Note that

This predicate behaves like findall/3 (it "relates to" the empty dict), not like bagof/3 (which fails if there are no solutions):

?- dict_keys(_{},Keys).
Keys = [].

instead:

?- Dict=_{}, bagof(Key-Value,get_dict(Key,Dict,Value),Bag).
false.

Missed opportunity

Add the possibility to pass a predicate to filter acceptable keys